Conventional processes for installation of window film on curved window surfaces such as windscreens typically involve a step of applying heat to a precut piece of film (known as a blank) using a heat gun or similar device. to shape the film blank to match the curvature of the window before it is applied to and installed on the window. For example, U.S. Pat. No. 6,304,720 generally describes processes for installing window film on automotive glass with compound curves wherein the film is heat-formed on the exterior surface of the glass so as to smoothly lay on the compound curvature to be subsequently adhesively applied to the inside surface. Similarly, U.S. Pat. No. 5,294,272 describes a method for applying a film to a surface of a contoured car window wherein the film is positioned on the outer surface of the window and subjected to localized heating to selectively shrink the film to match the window contour prior to installation of the film using an adhesive.
As further described in U.S. Pat. No. 6,304,720, the details of the heat-forming step typically involve the installer moving the heating device around and over the film surface at manually controlled distances and times, attempting to apply heat in an amount just sufficient to allow the film to form to the curved shape section by section in stepwise fashion without damaging the film. This aspect of the installation technique requires a substantial learning curve that is traversed mostly through trial and error, often resulting in substantial film damage and waste.
Films with ceramic layers, in particular solar control window films with titanium nitride layers such as for example those films sold by Eastman Chemical Company under the trademark Hüper Optik® and described in U.S. Pat. Nos. 6,188,512 and 8,404,303, the disclosures of which are incorporated herein by reference, can be particularly sensitive to negative impact from this heat-forming step. Even with proper heat-forming technique, ceramic layer-containing films and titanium nitride layer-containing films in particular have been known to exhibit a tendency to “ghost” or “whitewash” whereby a light, whitish haze would develop over certain random portions of the film during the heat-forming step in the installation process. “Ghosting” occurs when the titanium nitride layer or layers of the film construction crack during the heat-forming step and is believed to be caused by thermal stresses that develop from uneven shrinkage of the various materials in the different film construction layers. Though ghosting does not impact the solar control and heat rejection performance of the film when employed for these purposes, it is a cosmetic defect that is visible at certain angles and is therefore undesirable.
To date, installers have found (with limited success) that ghosting may be reduced through use of relatively lower heat-forming temperatures (e.g. 260° C. to 350° C.) and carefully controlled heat application variables such as heat gun distance, angle, motion, blower speed, diffusion and sequential zone treatment to effect the heat-forming step. Unfortunately, control of these processing conditions is highly dependent on installer skill level and equipment sophistication and may be beyond the capabilities of many window film installation service providers and shops. Further, ghosting has been found to occur even under processing conditions which are closely monitored and controlled.
A continuing need therefore exists in the art for a titanium nitride film that may be easily and effectively installed on contoured window surfaces using a wide variety of heat-forming conditions with minimal risk of ghosting or other negative effects from the installation process. A related need in the art is for a titanium nitride film installation method that minimizes if not eliminates the risk of ghosting and other negative effects when the film is applied to a contoured substrate.